The present invention relates to a static magnetic field generating apparatus used in a magnetic resonance imaging apparatus (hereafter referred to as MRI apparatus) and the like, and in particular to a magnetic field correction means for improving the field homogeneity of the static magnetic field generated by a static magnetic field generating apparatus.
Furthermore, the present invention relates to a superconducting magnet apparatus having a wide opening to provide a subject with feeling of openness and facilitate access to the subject.
Furthermore, the present invention relates to a superconducting magnet apparatus which can be constructed with ease and high precision after it has been carried in a site of use such as a hospital or a clinic.
In the case of MRI apparatuses, characteristics of the static magnetic field are the most important characteristics exert directly upon the image distortion, image blurredness, and signal-to-noise ratio, and the characteristics of the static magnetic field are the most important characteristics determining the fundamental performance in the MRI apparatus. Therefore, the static magnetic field in an imaging space (also called measurement space) is required to have high homogeneity and high stability of several ppm order. In order to generate a highly homogeneous and highly stable static magnetic field, a static magnetic field generating apparatus is used.
In the static magnetic field generating apparatus, there occurs a problem that a lot of magnetic fields leak outside the apparatus because a high static magnetic field is generated in the imaging space.
In order to reduce the leakage magnetic field, paths of lines of magnetic flux (magnetic path) generated from a magnetic field generating source are formed and the lines of magnetic flux are converged to the magnetic paths. By doing so, it is attempted to reduce the leakage of the magentic field to the outside of the apparatus.
An example of a conventional static magnetic field generating apparatus is shown in FIG. 21. FIG. 21 is an oblique view of the static magnetic field generating apparatus as a whole. In a static magnetic field generating apparatus 1 shown in FIG. 21, a high static magnetic field is generated in an imaging space 2 by static magnetic generating sources 3 disposed above and below the imaging space 2 so as to have the imaging space 2 between. Each of the static magnetic field generating sources 3 is formed by a superconducting coil 4 and a cooling vessel 5 for cooling the superconducting coil to a superconducting state. Magnetic circuits are formed so that lines Bz of magnetic flux generated in the vertical direction by the upper and lower superconducting coils 4 will be caught by platelike ferromagnetic substances 6 disposed above and below the static magnetic field generating sources 3 and returned to the superconducting coils 4 by using pillarlike ferromagnetic substances 7 disposed at the sides of the static magnetic field generating sources 3 as return paths. The pillarlike ferromagnetic substances 7 not only serve as the return paths of the lines of magnetic flux but also function to mechanically support structural components disposed above and below the imaging space 2. As materials of the platelike ferromagnetic substances 6 and the pillarlike ferromagnetic substances 7, iron is typically used from the viewpoint of the mechanical strength and cost price.
In the conventional static magnetic field generating apparatus of the horizontal magnetic field scheme, the magnetic circuits take the shape of a solenoid and the return paths are disposed symmetrically. Therefore, magnetic moments of the pillarlike portions are also uniform in the circumferential direction. In the case of the magnetic circuits having the return paths formed by the pillarlike ferromagnetic substances 7, however, distribution of the lines of magnetic flux in directions having the pillarlike ferromagnetic substance 7 differs from that in directions having no pillarlike ferromagnetic substance 7. Therefore, the magnetic field distribution in the circumferential direction varies. As a result, the static magnetic field in the imaging space 2 becomes inhomogeneous because of this variation of the magnetic field distribution in the circumferential direction. From the stage of design, therefore, correction of the inhomogeneous magnetic field components is considered. In the example of the conventional technique shown in FIG. 21, ferromagnetic substance pieces 8 are disposed on the surface of each of the cooling vessels 5 to conduct the magnetic field correction.
As a method for evaluating the magnetic field homogeneity, there is well known a method of determining a measuring point on a spherical surface to be evaluated in the static magnetic field, expanding the value of the magnetic field at the measuring point with the Legendre functions Pnm(cos xcex8) as shown in equation (1):                               B          Z                =                              ∑                          n              =              0                        ∞                    ⁢                                    ∑                              m                =                0                            n                        ⁢                                          r                n                            ⁢                                                P                  n                  m                                ⁡                                  (                                      cos                    ⁢                                          xe2x80x83                                        ⁢                    θ                                    )                                            ⁢                              "LeftBracketingBar"                                                                            A                      ⁡                                              (                                                  m                          ,                          n                                                )                                                              ⁢                    cos                    ⁢                                          xe2x80x83                                        ⁢                    m                    ⁢                                          xe2x80x83                                        ⁢                    φ                                    +                                                            B                      ⁡                                              (                                                  m                          ,                          n                                                )                                                              ⁢                    sin                    ⁢                                          xe2x80x83                                        ⁢                    m                    ⁢                                          xe2x80x83                                        ⁢                    φ                                                  "RightBracketingBar"                                                                        (        1        )            
where (r, xcex8, xcfx86) means a spherical coordinate system, r a distance from the coordinate center, xcex8 an angle formed with respect to the Z axis, and xcfx86 a rotation angle taken with center on the Z axis. Bz is a Z direction component magnetic flux density.
The smaller the value of the term A(m, n) in the equation (1) becomes as compared with the value of the static magnetic field term A(0, 0), the better the magnetic field homogeneity becomes. Hereafter, xe2x80x9cthe ratio of the coefficient of each expansion term to the term A(0, 0)xe2x80x9d is referred to as expansion coefficient ratio.
Components generated from the equation (1) can be broadly classified into xe2x80x9caxisymmetrical componentsxe2x80x9d and xe2x80x9cunaxisymmetrical components.xe2x80x9d The axisymmetrical components are magnetic field components which are symmetrical with respect to the Z axis (the static magnetic field direction). The axisymmetrical components depend on only the coordinates of (r, xcex8). Mathematically, the axisymmetrical components are terms of the Legendre functions with m=0. The unaxisymmetrical components depend on all coordinates of (r, xcex8, xcfx86). The unaxisymmetrical components are terms of the Legendre functions with mxe2x89xa00.
Typically as means for correcting the magnetic field homogeneity of the static magnetic field, ferromagnetic substance pieces (hereafter referred to as iron pieces because of use of iron) 8 are disposed on an opposed surface of opposed cooling vessels. The positions of the iron pieces 8 are determined so as to cancel the inhomogeneous magnetic field components. The greater the inhomogeneous magnetic field components are, therefore, the more amount of the iron pieces 8, i.e., the wider region for accommodating the iron pieces 8 is needed. As a result, the region (clear bore) for accommodating gradient magnetic coils (GCs) and a high frequency coil (RF coil) cannot be utilized effectively.
As a countermeasure against this, it is also conceivable to increase the region accommodating the iron pieces 8 by widening the distance between the opposed cooling vessels 5. If the distance between the opposed cooling vessels 5 is widened, however, the distance between opposed superconducting coils 4 is also increased. If the distance between opposed superconducting coils 4 is increased, magnetomotive force for generating a desired static magnetic field intensity increases in proportion to the third to fifth power of the distance. The increase in the magnetomotive force causes a complicated mechanical structure and an increased cost, resulting in a serious problem.
As heretofore described, the conventional magnetic field generating apparatus had a problem that an attempt to correct the inhomogeneous magnetic field in the static magnetic field caused an increased region for correction, an increased size of external shape of the apparatus, and an increased manufacturing cost.
An object of the present invention is to provide such a static magnetic field generating apparatus that the total weight of the iron pieces for correcting the static magnetic field and the region accommodating the iron pieces are confined to the minimum and the magnetic field homogeneity is favorable.
Furthermore, in the present invention, the contrivance for facilitating access to the subject is also made.
In the conventionally used static magnetic generating apparatus of the horizontal magnetic field scheme taking the shape of a cylinder, a measurement space accommodating the subject for imaging is narrow and is surrounded in circumference, and consequently the subject is provided with the feeling of blockade. Sometimes, therefore, the subject refused to enter the apparatus. Furthermore, it was also difficult for an inspector to access the subject from the outside of the apparatus.
As a technique for avoiding the above described problem, a superconducting magnet apparatus having a configuration shown in FIGS. 22 and 23 is publicly known. The magnet of an example of the conventional technique is disclosed in U.S. Pat. No. 5,194,810. The magnetic field in the vertical direction is generated by superconducting coils 4 disposed in coolant vessels disposed above and below. As for the coolant vessels, outside vacuum vessels 31 are shown in FIGS. 22 and 23. Inside this superconducting coil 4, magnetic field homogenizing means 32 formed by a ferromagnetic substance is provided in order to obtain favorable magnetic field homogeneity. In addition, iron plates 33 and iron yokes 34 are provided as the return paths for the magnetic flux generated by the upper and lower superconducting coils 4. Furthermore, the iron yokes 34 not only serve as the paths of the magnetic flux but also function to support the upper and lower structural substances.
In this example, the subject has no feeling of blockade because of openness on four sides. The inspector can also access the subject easily. Furthermore, since the iron yokes 34 provide the return paths of the magnetic flux, the magnetic flux does not spread far away and the magnetic field leakage can be reduced.
On the other hand, iron used as the magnetic field homogenizing means 32 has a hysteresis characteristic with respect to the magnetic field. Therefore, the pulsative magnetic field generated by the gradient magnetic field coils (not illustrated) disposed near the magnetic field homogenizing means 32 exerts an influence upon the magnetic field distribution of the magnetic field homogenizing means 32. Since this influences even the magnetic field distribution inside the homogeneous magnetic field generation region 2, there is a possibility of high precision signal measurement being hindered. Against this, means such as use of a material having low electric conductivity for the magnetic field homogenizing means 32 has been adopted. In the case where the intensity of the pulsative magnetic field is intense, however, a sufficient effect cannot be obtained.
Furthermore, the magnetization characteristic (B-H characteristic) has dependency upon the temperature. If the temperature of iron varies, therefore, it becomes a factor of a variation of the magnetic field homogeneity which is an important factor for the MRI apparatus. In the structure of FIG. 23, it is typical to install the gradient magnetic field coils near the magnetic field homogenizing means 32. Since the magnetic field homogenizing means 32 is heated by heat generated by driving the gradient magnetic field coils, its temperature is apt to vary.
As an example of a configuration eliminating the problems of the above described example of the conventional technique, a structure shown in FIGS. 24 and 25 has been proposed. This structure has been proposed in JP-A-8-19503 (PCT/JP94/00039). FIG. 24 is an oblique view of a superconducting magnetic apparatus as a whole. FIG. 25 is a longitudinal section view thereof. Supposing a NbTi wire material usually often used as a coil wire material, a coolant vessel 35 storing liquid helium is provided in order to cool the superconducting coil 4. Above and below a homogeneous magnetic field generation region 2 located in the center of the apparatus, circular superconducting coils 4 are disposed symmetrically in the vertical direction. According to them, cylindrical cooling vessels 36 each containing a cooling vessel 35 and a vacuum vessel 31 are disposed symmetrically in the vertical direction, and are held by coupling tubes 37 located between them so as to keep a predetermined distance between them.
In this example of the conventional technique, there is provided such a structure as to effectively reduce the leakage magnetic field generated outside the apparatus by the above described superconducting coils 4 by using ferromagnetic substances arranged in the peripheral part of the apparatus. In other words, the periphery of each of the cooling vessels 36 is surrounded by an iron container 40 formed by a circular iron plate 38 and an iron cylinder 39 as shown in FIG. 25. The upper and lower iron container 40 are connected by iron columns 41. Thereby, a magnetic path for the magnetic flux generated outside the apparatus is formed. Therefore, it is possible to prevent the leakage magnetic field from spreading far away.
On the other hand, in this structure, the iron columns 41 and the coupling tubes 37 are present in the transverse direction with respect to the homogeneous magnetic field generation region 2 serving as the measurement space. Therefore, there is a problem that the access to the subject from the outside of the apparatus is difficult. Furthermore, there is a problem that the subject feels oppression because the subject is in the magnet and the view in the transverse direction is obstructed.
As for the configuration of an MRI apparatus facilitating the inspector""s access to the subject from the flank of the magnet, there is a configuration disclosed in JP-A-8-50170. In this magnet, permanent magnets 50 are disposed above and below a homogeneous magnetic field generation region 2 as shown in FIG. 26. In order to support the permanent magnets 50 and form a magnetic path 51 serving as the return path of the magnetic flux generated by the permanent magnets 50, platelike first yokes 52 and columnar second yokes 53 are provided. The columnar second yokes 53 are disposed on the rear side with respect to the homogeneous magnetic field generation region 2. As a result, an opened measurement space (homogeneous magnetic field generation region) 2 is formed frontward. In this configuration, the subject is released from the feeling of oppression and the inspector""s access to the subject from the outside of the apparatus is facilitated.
In this magnet, however, the permanent magnets 50 are used. This results in a problem that it is difficult to increase the magnetic field intensity in the homogeneous magnetic field generation region 2, and, in the case of high magnetic field, it is also difficult to decrease the magnetic field leakage outside the apparatus.
As described above, it was impossible to access the subject from the flank of the magnet in the conventional static magnetic field generating apparatus in which the magnetic field in the vertical direction was applied to the subject and magnetic shielding was effected. Therefore, giving treatment to the subject and coping with the IVMR (Interventional MR) vigorously developed in recent years were also difficult.
An MRI apparatus using permanent magnets facilitating the access to the subject has been proposed. In this MRI apparatus, however, it is difficult to achieve a high magnetic field and it is necessary to further improve the magnetic field intensity to realize a high picture quality.
Therefore, an object of the present invention is to provide a static magnetic generating apparatus capable of solving the above described problems, making it possible to access the subject from the flank of the magnet, lightening the feeling of oppression of the subject, and applying a magnetic field of high intensity to the subject in the vertical direction.
Furthermore, in the present invention, such a contrivance as to facilitate the carrying in and installation of superconducting magnets has been made.
FIG. 22 shows an example of a superconducting magnet apparatus for MRI apparatus using a magnetic shield made of iron. As the magnetic field intensity is increased, however, the superconducting magnet apparatus for MRI apparatus having such a structure tends to become large in size and weight. In order to supply a superconducting magnet apparatus to a hospital or the like, therefore, it is necessary to first disassemble the apparatus in the factory so far as it can be disassembled and assemble the apparatus on the spot. Or it is necessary to, for example, enlarge or rebuild a building so as to be suitable for assembly and installation of the superconducting magnet apparatus, or dividing the superconducting magnet apparatus into components to such a degree that the components can pass the entrance and passages of the building, carrying the components in the building, and then assembling the apparatus in a shield room. From the viewpoint of the cost, the latter cited divided carrying in and assembly becomes more favorable. In the case where the divided carrying in and assembly method has been adopted, easiness of the carrying in, easiness of the assembly, and high assembly precision are demanded.
Considering the carrying in and assemble method for the above described conventional apparatuses supposing this demand, First of all, the space between the iron yokes 34 is narrower than the diameter of the superconducting magnet. After the magnetic shield has been assembled, therefore, it is impossible to insert the superconducting magnets from the transverse direction. At the time of assembly of the apparatus, therefore, there are conceivable methods such as (a) a method of assembling the lower iron plate 33, placing the superconducting magnets thereon, and then putting the iron yokes 34 and the upper iron plate 33 together, or (b) a method of putting the lower iron plate 33 and the iron yokes 34, inserting the superconducting magnets from the top, and placing the upper iron plate 33.
Furthermore, in this example of the conventional technique, the annular magnetic field homogenizing means 32 is fixed to such a face of the iron plate 33 of the magnetic shield as to be opposed to the measurement space 2 for the purpose of correcting the magnitude field distribution in the measurement space 2. After inserting the superconducting magnets in the magnetic shield and fixing the superconducting magnets, therefore, these magnetic field homogenizing means 32 must be fixed, resulting in a very poor work efficiency.
For carrying the large-sized and heavy superconducting magnet apparatus for MRI apparatus in the place of use such as a hospital, it is necessary to manufacture the apparatus divisionally and assemble the apparatus on the spot as described with reference to the aforementioned conventional technique. Even if the magnetic shield is divided into several parts in the divisional carrying in and on-the-spot assembling method, however, it was necessary to assemble the magnetic shield, incorporate the superconducting magnets in the magnetic shield, and thereafter put the upper iron plate 33 or the iron yokes 34. Assembly using this method is not an easy work for the magnetic shield made large in size. In parallel to assembling of the superconducting magnets each having a weak strength, the heavy magnetic shield must be assembled. Furthermore, in the case where the superconducting magnet is divided and assembled on the spot, the reproducibility of the magnetic field adjustment in the factory is not maintained and fine adjustments were needed on the spot.
In order to solve such problems, an object of the present invention is to provide such a superconducting magnet apparatus that the yoke portions of the magnetic shield are divided into small components and after the magnetic shield is assembled on the spot the superconducting magnets can be easily incorporated therein.
In order to achieve the above described object of improving the magnetic field homogeneity of the static magnetic field, a static magnetic field generating apparatus including static magnetic field generating sources for generating a static magnetic field of a first direction in a finite region, a pair of magnetic flux converging means disposed so as to nearly perpendicular to the first direction to converge the lines of magnetic flux generated by the static magnetic field generating sources, and at least one magnetic flux passing means for magnetically coupling the pair of magnetic flux converging means and forming a magnetic path letting flow the lines of magnetic flux has such a configuration according to the present invention that magnetic field correcting means for effecting a magnetic field correction of the static magnetic field of the finite region is provided in a portion of the magnetic flux converging means, and the magnetic field correcting means has one or more portions filled with a material of a low relative permeability and/or one or more air gap holes.
In this configuration, portions filled with a material of a low relative permeability and/or air gap holes serving as the magnetic field correcting means are provided in a part of the magnetic flux converging means. Thereby, the flow of the lines of magnetic flux in the magnetic flux converging means is changed. As a result, the static magnetic field in the finite region serving as the imaging space is subject to the magnetic field correction.
Furthermore, in the static magnetic field generating apparatus of the present invention, the portions filled with the material of the low relative permeability or air gap holes are circular, and are disposed nearly in a central portion of the magnetic flux converging means nearly along the first direction.
In this configuration, the magnetic field correcting means are circular. This brings about effects of fine workability, cost reduction, and a large correction effect.
Furthermore, in the static magnetic field generating apparatus of the present invention, the portions filled with the material of the low relative permeability or air gap holes are elliptical or rectangular, and are disposed nearly in a central portion of the magnetic flux converging means nearly along the first direction. Furthermore, the magnetic flux passing means are disposed on the left side and the right side with respect to the central axis running parallel with the first direction of the static magnetic field generating sources, and a longitudinal direction of the elliptical or rectangular portions or air gap holes is nearly made parallel to a direction of presence of the magnetic flux passing means.
In this configuration, the magnetic field correcting means are elliptical or rectangular, and their longitudinal direction is parallel to the side of presence of the magnetic flux passing means. Therefore, the change of flow of the lines of magnetic flow is large, and the magnetic correction effect is large.
Furthermore, in the static magnetic field generating apparatus of the present invention, a plurality of portions filled with the material of the low relative permeability and/or air gap holes are disposed in positions radially located away from a central axis, the central axis passing through a center of the static magnetic field and being parallel to the first direction. Furthermore, the portions filled with the material of the low relative permeability and/or air gap holes are fan-shaped or circular, or disposed in positions symmetrical with respect to the central axis.
In this configuration, the magnetic field correcting means are disposed in positions radially located away from the central axis. Therefore, it is effective in local magnetic field correction. Furthermore, providing a plurality of magnetic field correcting means in symmetrical positions makes possible a large magnetic field correction.
Furthermore, in the static magnetic field generating apparatus of the present invention, one or more out of the air gap holes are tapped holes, and bolts each formed entirely or partially by a ferromagnetic substance are inserted into one or more of the tapped holes.
In this configuration, a bolt made of a ferromagnetic substance is inserted into each of the tapped holes serving as the magnetic field correcting means. The magnetic field correction is conducted by adjusting the quantity of the ferromagnetic substance present in the tapped hole portion. Fine adjustment of the magnetic field thus becomes possible. If the quantity of the ferromagnetic substance is increased, the correction quantity of the magnetic field becomes large. If the quantity of the ferromagnetic substance is decreased, the correction quantity of the magnetic field becomes small.
Furthermore, in the static magnetic field generating apparatus of the present invention, the portions filled with the material of the low relative permeability and/or air gaps are subject to working for changing the area of the section, such as taper working or step working, in the depth direction.
In this configuration, the taper working or step working is conducted. As compared with the case of simple holes or the like, therefore, magnetic field correction having a larger adjustment width can be effected.
Furthermore, in the static magnetic field generating apparatus of the present invention, the diameter of the circular portions, the major diameter and minor diameter of the elliptical or rectangular portions, the number, positions, and size of the portions filled with the material of the low relative permeability and/or the air gap holes, the number and positions of the tapped holes, the quantity of a ferromagnetic substance inserted into the tapped holes, and the area of the section of the taper working portion or step working portion are determined so as to be related to the flow of lines of magnetic flux in the magnetic flux converging means.
The change of the above described parameters of the magnetic field correcting means causes a change of the flow of the lines of magnetic flux in the magnetic flux converging means and consequently the magnetic field correction of the static magnetic field is conducted. In view of this fact, the above described parameters are determined, in this configuration, so as to be related to the flow of lines of magnetic flux in the magnetic flux converging means.
Furthermore, the static magnetic field generating apparatus of the present invention uses a superconductor as current carrying means, and uses a cooling vessel containing a coolant for cooling the superconductor to a superconducting state and maintaining the superconductor in the superconducting state, and includes a refrigerator for cooling the coolant. Furthermore, the refrigerator is disposed in the portion filled with the material of the low relative permeability or in an air gap hole.
In this configuration, a static magnetic field of high intensity and high homogeneity is implemented by using static magnetic field generating sources of a superconducting system. In addition, the magnetic correcting means is used efficiently as a place for attaching the refrigerator.
A magnetic resonance imaging apparatus of the present invention uses the above described static magnetic field generating apparatus of the present invention.
According to the present invention, the magnetic field correction of the static magnetic field in the imaging space can be conducted on the platelike ferromagnetic substance as heretofore described. Therefore, it is possible to decrease sharply the quantity and disposition area of the iron pieces disposed near the imaging space to correct the magnetic field and keep them at the minimum. In addition, the homogeneity of the magnetic field can also be improved. As a result, a static magnetic field generating apparatus having a wide imaging space and a favorable magnetic field homogeneity can be provided.
In order to achieve the above described object of facilitating the access to the subject, a superconducting magnet apparatus including static magnetic field generating sources for letting flow a current to generate a homogeneous magnetic field of a vertical direction in a finite region, the static magnetic field generating sources being formed by a material having superconducting characteristics, cooling means for cooling the static magnetic field generating sources to such a temperature that superconducting characteristics are exhibited and keeping the static magnetic field generating sources at the temperature, and support means for supporting the static magnetic field generating sources, wherein the static magnetic field generating sources are formed by two sets of magnetic field generating means having nearly same shape disposed in positions located at nearly equal distances from a center of the homogeneous magnetic field generation region along the vertical direction with the homogeneous magnetic field generation region between, has such a configuration in accordance with the present invention that the support means are disposed on the rear side with respect to the central axis of the vertical direction passing the center of the homogeneous magnetic field generation region.
In this configuration, the magnetic field generating sources are disposed so as to be opposed to each other in the vertical direction, and the support means for supporting the magnetic field generating sources are disposed on the rear side with respect to the central axis of the homogeneous magnetic field generation region. Therefore, a high magnetic field can be generated in the homogeneous magnetic field generation region. In addition, the front and flanks of the homogeneous magnetic field generation region serving as the measurement space are open. Therefore, the inspector can easily access the subject from a side of the apparatus.
Furthermore, the static magnetic field generating apparatus of the present invention includes first ferromagnetic substances respectively surrounding peripheries of the magnetic field generating sources, and second ferromagnetic substances for magnetically connecting the first ferromagnetic substances. The second ferromagnetic substances have both ends located in nearly the same positions as those of the first ferromagnetic substances. The second ferromagnetic substances are disposed so as to be located on the rear side with respect to the central axis of the vertical direction together with the support means.
In this configuration, the first ferromagnetic substances are disposed around the magnetic field generating sources, and the first ferromagnetic substances are magnetically coupled by the second ferromagnetic substances. Thereby, an external magnetic path (return path) of the magnetic flux generated by the magnetic field generating sources is formed. The leakage magnetic field in the outside of the apparatus is thus reduced. Furthermore, the second ferromagnetic substances are disposed on the rear side with respect to the central axis of the homogeneous magnetic field generation region in the same way as the support means of the magnetic field generating sources. Therefore, the inspector can easily access the subject from a side of the apparatus.
Furthermore, in a preferred aspect of a superconducting magnet apparatus according to the present invention, the support means have a nearly circular section shape, and the support means are disposed so that at least one support means will be present on each of left and right sides with respect to the central axis of the vertical direction. Furthermore, the support means are disposed so that two or more support means will be located on each of left and right sides with respect to the central axis of the vertical direction, and the distance between support means included in the support means and disposed on the rear side with respect to the central axis is shorter than the distance between support means included in the support means and disposed on the front side with respect to the central axis.
Furthermore, in a preferred aspect of a superconducting magnet apparatus according to the present invention, the support means have a nearly rectangular or elliptical section shape, and the support means are disposed so that at least one support means will be present on each of left and right sides with respect to the central axis of the vertical direction.
Furthermore, in a preferred aspect of a superconducting magnet apparatus according to the present invention, a viewing angle viewing the support means when viewed from the central axis of the vertical direction is smaller than a viewing angle viewing the second ferromagnetic substance when viewed from the central axis of the vertical direction. More preferably, a viewing angle viewing the support means located on the left side or the right side as a whole when viewed from the central axis of the vertical direction is 60 degrees or less.
Furthermore, in a preferred aspect of a superconducting magnet apparatus according to the present invention, the shortest distance between the support means disposed on the left side with respect to the central axis of the vertical direction and the support means disposed on the right side with respect to the central axis of the vertical direction is at least 600 mm.
Furthermore, in a preferred aspect of a superconducting magnet apparatus according to the present invention, the second ferromagnetic substances are disposed in nearly the same positions in the peripheral direction as the support means with respect to the central axis of the vertical direction. Furthermore, faces of the second ferromagnetic substances opposed to the support means are formed so as to be nearly flat. Furthermore, the flat face of each of the second ferromagnetic substances is nearly perpendicular to the line coupling the central axis of the vertical direction to the central axis of the second ferromagnetic substance. Furthermore, at least one auxiliary iron member is added to the flat faces formed on the second ferromagnetic substances.
Furthermore, in a preferred aspect of a superconducting magnet apparatus according to the present invention, the support means and the second ferromagnetic substances are disposed so that a line touching a periphery of support means included in the support means disposed on the left side and on the right side with respect to the central axis of the vertical direction and located on the most front side at one point will be located in a position more rear than a line touching a periphery of a second ferromagnetic substance included in the ferromagnetic substances disposed in the same way and located on the most front side at one point.
Furthermore, in a preferred aspect of a superconducting magnet apparatus according to the present invention, two faces included in peripheral faces of each of the second ferromagnetic substances are formed so as to be nearly parallel to two tangential lines drawn from the central axis of the vertical direction to the peripheral faces of the second ferromagnetic substance.
Furthermore, in a preferred aspect of a superconducting magnet apparatus according to the present invention, covers each containing the support means and the second ferromagnetic substance are provided.
Furthermore, in the present invention, a static magnetic field generating apparatus having the above described configuration is used as a static magnetic field generating apparatus of an MRI apparatus.
By forming a static magnetic field generating apparatus as described above, it becomes possible as heretofore described to provide a static magnetic field generating apparatus which allows accessing the subject accommodated in the measurement space from a side of the apparatus, which mitigates the feeling of oppression of the subject, and which applies a high magnetic field to the subject in the vertical direction.
In order to achieve the above described object of achieving carrying in and installation, a superconducting magnet apparatus of the present invention including a combination of a superconducting magnet and a magnetic shield, the superconducting magnet including a static magnetic field generating source for generating a homogeneous magnetic field of a vertical direction in a finite region, the static magnetic field generating source being formed by a material having superconducting characteristics, cooling means for housing the static magnetic field generating source, cooling the static magnetic field generating source to such a temperature that superconducting characteristics are exhibited and keeping the static magnetic field generating source at the temperature, support means for supporting the static magnetic field generating sources, the magnetic shield being formed by platelike ferromagnetic substances disposed above and below the superconducting magnet so as to surround the periphery of the superconducting magnet, and a plurality of columnar ferromagnetic substances for magnetically coupling and supporting the platelike ferromagnetic substances, has such a structure that the magnetic shield has at least one opening portion wider than the superconducting magnet. After assembling the magnetic shield, therefore, the superconducting magnet can be inserted into the magnetic shield and incorporated therein.
In this configuration, a superconducting magnet apparatus is formed by a superconducting magnet and a magnetic shield. After the magnetic shield has been assembled, the superconducting magnet can be inserted into the magnetic shield and incorporated therein. In the case where the superconducting magnet apparatus is to be supplied to a hospital or the like, therefore, it is possible to assemble and adjust the superconducting magnet apparatus in a factory, then disassemble the superconducting magnet apparatus into the superconducting magnet and the components of the magnetic shield, carry them to the site, thereafter assemble the magnetic shield, insert the superconducting magnet into the magnetic shield, and assemble and adjust the entire apparatus. As a result, divisional carrying in and on-the-spot assembling become possible, and the cost can also be reduced.
Furthermore, in a superconducting apparatus of the present invention, the columnar ferromagnetic substances are disposed so as not to overlie an insertion locus of the superconducting magnet obtained when inserting the superconducting magnet into the magnetic shield. In this configuration, there is no columnar ferromagnetic substance on the path for inserting the superconducting magnet into the magnetic shield. As a result, the superconducting magnet can be smoothly inserted.
Furthermore, in a superconducting apparatus of the present invention, the space between at least two columnar ferromagnetic substances included in the above described columnar ferromagnetic substances is equal to at least such a dimension that the superconducting magnet can be inserted into the magnetic shield. In this configuration, the superconducting magnet can be inserted into the magnetic shield through a space between two columnar ferromagnetic substances of the magnetic shield.
Furthermore, in a superconducting apparatus of the present invention, guide means for inserting the superconducting magnet into the magnetic shield is provided on contact faces of the superconducting magnet and the magnetic shield in the insertion direction of the superconducting magnet. In this configuration, the guide means for inserting the superconducting magnet into the magnetic shield is provided on contact faces of the superconducting magnet and the magnetic shield. By using the guide means, therefore, the superconducting magnet can be inserted into a proper position in the magnetic shield.
In an aspect of the guide means, convex/concave faces fitting in with those of another contact face are provided parallel to the insertion direction of the superconducting magnet. Furthermore, in another aspect of the guide means, one or more grooves are provided on one or both of the contact faces of the superconducting magnet and the magnetic shield so as to be parallel to the insertion direction of the superconducting magnet.
Furthermore, in a superconducting magnet apparatus of the present invention, the superconducting magnet is inserted into the magnetic shield, and thereafter each of the grooves is filled up by a solid or liquid substance containing a ferromagnetic substance. In this configuration, each of the grooves which were used as the guide means for insertion of the superconducting magnet is filled up by a solid or liquid substance containing a ferromagnetic substance. As a result, the close adherence between the superconducting magnet and the magnetic shield is improved, and the correction of the magnetic field distribution in the measurement space can be conducted.
Furthermore, in a superconducting magnet apparatus of the present invention, one or more gap portions are provided in a contact portion between the superconducting magnet and the magnetic shield, and a solid or liquid substance containing a ferromagnetic substance is disposed in the gap portions. In the case of this configuration as well, there is obtained an effect similar to that obtained in the case where each of the grooves is filled up by a solid or liquid substance containing a ferromagnetic substance.
Furthermore, in a superconducting magnet apparatus of the present invention, a ferromagnetic piece for magnetic field distribution correction is added to a portion of the superconducting magnet touching the magnetic shield so as to be integral with the superconducting magnet, and the superconducting magnet with the ferromagnetic piece added thereto is inserted into the magnetic shield. In this configuration, the ferromagnetic piece for magnetic field distribution correction can be added to the superconducting magnet beforehand, and a resultant integrated superconducting magnet can be incorporated into the magnetic shield. As a result, the on-the-spot assembling and adjustment for magnetic field distribution correction are facilitated.
Furthermore, in the present invention, the superconducting magnet apparatus having the above described configuration is used as the superconducting magnet apparatus of a MRI apparatus.
As heretofore described, it becomes possible according to the present invention to reduce the labor required for assembling and adjustment work at the time of supply of a superconducting magnet apparatus for MRI apparatus, reduce the manufacturing cost owing to the simplified structure, and provide a superconducting magnet apparatus for MRI apparatus improved in magnetic field homogeneity by utilizing the gap between the superconducting magnet and the magnetic shield.